The present invention relates to variable gain circuits and, more particularly, to an improved circuit and method for reducing offset errors in a variable gain circuit.
A charge-couple device (CCD) is the sensor of choice in modern imaging to convert photons into electrons, hence enabling the use of electronics for image processing. FIG. 1 shows typical analog front-end building blocks for a CCD signal processing channel. The CCD input signal 100 is received by a correlated double sampling circuit (CDS) 102 whose function is to extract the image content from the CCD signal 100 and remove an unwanted correlated noise component. The output of the CDS 102 is amplified by a programmable gain amplifier (PGA) 104 before it gets converted to digital data by an analog-to-digital converter (ADC) 106. The gain of the PGA can be programmed by providing a gain input to the gain control circuit which controls the gain in the PGA.
In reality, the circuit building blocks have offset, and such offset can reduce the dynamic range of the processing channel. In particular, an offset input to the PGA can get amplified by the PGA by an amount of the gain of the PGA, and hence seriously reduce the useful dynamic range of the PGA output and ADC. Such an offset can come from the CCD signal, the CDS, or can be the input referred offset of the PGA. For example, an offset of 10 mV input into a PGA with a gain of 50xc3x97, yields an output referred offset at the output of the PGA of 0.5V. This reduces the dynamic range of the PGA output and ADC by 0.5V, which is not acceptable in most integrated circuit design applications.
In order to address this problem, an offset correction is typically used. One way to provide an offset correction is to integrate the output of the PGA and subtract the accumulated error from the input of the PGA in a feedback fashion. The feedback will adjust the input of the PGA such that the output of the PGA is equal to the system""s xe2x80x9czeroxe2x80x9d reference. This scheme is shown in FIG. 2. In this figure, INT 200 refers to an integrator.
One problem with the scheme of FIG. 2 is that the time constant of the loop depends on the gain of the PGA 104. To keep the feedback loop stable and the noise of the xe2x80x9czeroxe2x80x9d reference low, it is preferable to keep the bandwidth of the loop low and constant, thereby keeping the loop gain constant with varying PGA gain. This can be accomplished by inserting another PGA in the feedback path with a reciprocal gain characteristic of the PGA in the forward path. This is called a reverse PGA (RPGA) 300, which is shown in FIG. 3. The gain characteristics of PGA and RPGA and the loop gain are shown in FIG. 4. In FIG. 4, shown are the gain of the RPGA, the gain of the PGA, and the loop gain, each with respect to the gain input provided to the gain control circuit. In terms of the dynamics of the loop, the order of RPGA 300 and INT 200 in the feedback path does not matter, but it does have circuit level consequences that usually limit the circuit to having INT 200 on the output side of the PGA 104.
One problem with the modified scheme of FIG. 3 is that any offset at the input side of the PGA 104 gets gained back to the input of the RPGA 300 or the output of the INT 200. This offset can consist of the CCD""s offset, the CDS"" offset, or the input referred offset of the PGA, and can be substantial. Although this offset does not directly affect the dynamic range of the processing channel itself (CDS, PGA, and ADC), it severely limits the operation of the RPGA and INT when the gain of the PGA is high. Again, an offset of 10 mV with a 50xc3x97 gain in the PGA gets referred back to the input of RPGA as 0.5V. Under a large offset condition, the offset correction scheme of FIG. 3 fails to operate due to the limited headroom range of the RPGA 300 and INT 200.
One embodiment of the invention is directed to a variable gain circuit that includes a first PGA in a feedforward signal path and a second PGA connected in feedback with the first PGA. The variable gain circuit need not be used in an image sensor application (e.g., CCD) as disclosed herein, but may be any variable gain circuit in which it is desired to reduce offset errors of the circuit. An embodiment of the invention includes circuitry for controlling the gain of the first PGA and controlling the gain of the second PGA, independently of one another.
Another embodiment of the invention is directed to a method for correcting offset errors in a variable gain circuit including a first PGA and a second PGA connected in feedback with the first PGA. The method includes the steps of: controlling a gain of the first PGA, controlling a gain of the second PGA independently from the gain control of the first PGA.